Boilers are classified into a smoke pipe type and a water pipe type. The smoke pipe type generally used for household or small-sized combustion-type boilers is configured such that high-temperature combustion gas flows in a boiler pipe and the outside of the pipe is surrounded with water. A smoke-pipe-type boiler is light and can be easily handled. However, due to a limit of the outside area of the pipe, the heat transfer amount is small and thus heat efficiency is low.
Meanwhile, the water-pipe-type boiler generally applied to high-capacity boilers for ships or industrial field is configured such that water exists inside a boiler pipe and combustion gas is brought into contact with the outside of the pipe for heat transfer. Therefore, the water-pipe-type boiler has higher heat efficiency than the smoke-pipe-type boiler. However, the weight and size are large and thus the water-pipe-type boiler is not suitable for being applied to a general steam car washer.
Also, in a general steam boiler, cool air (oxygen) is injected into a combustion chamber. However, in this case, the combustion efficiency is very low. In particular, a low atmospheric temperature in a cold country or during winter continuously causes incomplete combustion such that smoke and soot are generated. Therefore, global warming problem due to fossil fuel may occur.
The common disadvantage of the general smoke-pipe-type and water-pipe-type boilers is that, for insulation, a heat insulating material should be necessarily additionally provided and attached to the outside of the main body of the boiler. As the heat insulating material, asbestos, plaster, glass fiber, or the like are used. However, these materials become a factor threatening health of a worker during manufacture, and may increase the manufacture cost and the volume and weight of the boiler.
The present invention proposes a new type of steam boiler taking all advantages of the smoke-pipe-type and the water-pipe-type boilers, and making it unnecessary to provide a heat insulating material to the main body of the boiler.
Further, waste heat of discharge gas is used to preheat cool air for combustion, and thus it is possible to further improve heat efficiency.